tests: reserve more fds for commandtest

tests: reserve more fds for commandtest

Commit 39c77fe triggered random failures, depending on the platform
and what other fds leak into the testsuite (for me, it passed on
RHEL 6 but failed on Fedora 18).  The reason was that we were
expecting an fd that fell outside of our reserved range.  By reserving
a larger range, the test once again passes on all platforms.

* tests/commandtest.c (mymain): Reserve enough fds.